Thoughts on hidden lore....

I swear I love writing. I have seen other writers hide stuff on the lore. I.e. stuff that was ether to adult for the rating the book was in or stuff very messed up but would ruin the books time. On top of hiding stuff in plane site that is in side the book. I was wondering your thoughts on that. Things like someone's demonic origins but leaving little hints what kind of demon they are.... Or a realationship with one person between two siblings who don't date each other... There just very trusting of their crush and each other... Those are some things I wrote about.